This invention relates generally to feedthrough capacitor terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers and the like, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to a method of providing a conductive coating on the flanges of human implantable hermetic seals for reliable EMI filter attachment, and a method of electrical connection of the feedthrough capacitor to the feedthrough lead wires at the hermetic gold braze. This invention is particularly designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia), neuro-stimulators, internal drug pumps, cochlear implants and other medical implant applications. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules, where it is desirable to preclude the entry of EMI into a hermetically sealed housing containing sensitive electronic circuitry.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In the prior art devices, the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device.
In a typical prior art unipolar construction (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor is combined with a hermetic terminal pin assembly to suppress and decouple undesired interference or noise transmission along a terminal pin. FIGS. 1-6 illustrate an exemplary prior art feedthrough filter capacitor 100 and its associated hermetic terminal 102. The feedthrough filter capacitor 100 comprises a unitized dielectric structure or ceramic-based monolith 104 having multiple capacitor-forming conductive electrode plates formed therein. These electrode plates include a plurality of spaced-apart layers of first or “active” electrode plates 106, and a plurality of spaced-apart layers of second or “ground” electrode plates 108 in stacked relation alternating or interleaved with the layers of “active” electrode plates 106. The active electrode plates 106 are conductively coupled to a surface metallization layer 110 lining a bore 112 extending axially through the feedthrough filter capacitor 100. The ground electrode plates 108 include outer perimeter edges which are exposed at the outer periphery of the capacitor 100 where they are electrically connected in parallel by a suitable conductive surface such as a surface metallization layer 114. The outer edges of the active electrode plates 106 terminate in spaced relation with the outer periphery of the capacitor body, whereby the active electrode plates are electrically isolated by the capacitor body 104 from the conductive layer 114 that is coupled to the ground electrode plates 108. Similarly, the ground electrode plates 108 have inner edges which terminate in spaced relation with the terminal pin bore 112, whereby the ground electrode plates are electrically isolated by the capacitor body 104 from a terminal pin 116 and the conductive layer 110 lining the bore 112. The number of active and ground electrode plates 106 and 108, together with the dielectric thickness or spacing therebetween, may vary in accordance with the desired capacitance value and voltage rating of the feedthrough filter capacitor 100.
The feedthrough filter capacitor 100 and terminal pin 116 is assembled to the hermetic terminal 102 as shown in FIGS. 5 and 6. In the exemplary drawings, the hermetic terminal includes a ferrule 118 which comprises a generally ring-shaped structure formed from a suitable biocompatible conductive material, such as titanium or a titanium alloy, and is shaped to define a central aperture 120 and a ring-shaped, radially outwardly opening channel 122 for facilitated assembly with a test fixture (not shown) for hermetic seal testing, and also for facilitated assembly with the housing (also not shown) on an implantable medical device or the like. An insulating structure 124 is positioned within the central aperture 120 to prevent passage of fluid such as patient body fluids, through the feedthrough filter assembly during normal use implanted within the body of a patient. More specifically, the hermetic seal comprises an electrically insulating or dielectric structure 124 such as a gold-brazed alumina or fused glass type or ceramic-based insulator installed within the ferrule central aperture 120. The insulating structure 124 is positioned relative to an adjacent axial side of the feedthrough filter capacitor 100 and cooperates therewith to define a short axial gap 126 therebetween. This axial gap 126 forms a portion of a leak detection vent and facilitates leak detection. The insulating structure 124 thus defines an inboard face presented in a direction axially toward the adjacent capacitor body 104 and an opposite outboard face presented in a direction axially away from the capacitor body. The insulating structure 124 desirably forms a fluid-tight seal about the inner diameter surface of the conductive ferrule 118, and also forms a fluid-tight seal about the terminal pin 116 thereby forming a hermetic seal suitable for human implant. Such fluid impermeable seals are formed by inner and outer braze seals or the like 128 and 130. The insulating structure 124 thus prevents fluid migration or leakage through the ferrule 118 along any of the structural interfaces between components mounted within the ferrule, while electrically isolating the terminal pin 116 from the ferrule 118.
The feedthrough filter capacitor 100 is mechanically and conductively attached to the conductive ferrule 118 by means of peripheral material 132 which conductively couple the outer metallization layer 114 to a surface of the ferrule 118 while maintaining an axial gap 126 between a facing surface of the capacitor body 104, on the one hand, and surfaces of the insulating structure 124 and ferrule 118, on the other. The axial gap 126 must be small to preclude leakage of EMI. The outside diameter connection between the capacitor 100 and the hermetic terminal ferrule 118 is accomplished typically using a high temperature conductive thermal-setting material such as a conductive polyimide. It will also be noted in FIG. 5 that the peripheral support material 132 is preferably discontinuous to reduce mechanical stress and also allow for passage of helium during hermetic seal testing of the complete assembly. In other words, there are substantial gaps between the supports 132 which allow for the passage of helium during a leak detection test.
In operation, the coaxial capacitor 100 permits passage of relatively low frequency electrical signals along the terminal pin 116, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six) and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal, such as titanium alloy which is electrically and mechanically coupled to the hermetic terminal pin assembly which in turn is electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
It is well known in the art that titanium has a tendency to form oxides, particularly at high temperature. Titanium oxide (or trioxide) is typical of the oxides that form on the surfaces of titanium. Titanium oxide is very rugged and very stable and in fact is often used as a pigment in paints due to its long-term stability. It is also an insulator or semiconductor.
In the prior art, the attachment between the capacitor outside diameter metallization 114 and the titanium ferrule 118 is accomplished using a thermalsetting conductive adhesive 132, such as a conductive polyimide. Ablestick Corporation manufactures such polyimide compounds. If the oxide layer 134 builds up sufficiently in thickness, this can form an insulative surface which can preclude the proper operation of the feedthrough capacitor 100 as an effective electromagnetic interference filter. It is essential that the capacitor ground electrode plates 108 have a very low resistance and low impedance connection at RF frequencies. This is essential so that it can perform as a proper high frequency bypass element (transmission line) which will short out undesirable electromagnetic interference such as that caused by cellular telephones and other emitters. If the oxide layer 134 is very thin, it creates only a few milliohms of extra resistance. However, recent measurements indicate that a thicker oxide layer can create resistance (measured at 10 MHz) ranging from 750 milliohms to over 30 ohms.
In the past, this oxide layer 134 was very difficult to detect with conventional measuring instruments. Agilent Technologies has recently produced a new piece of equipment known as the E4991A Materials Analyzer. This materials analyzer has the capability to measure equivalent series resistance and other properties of capacitors at very high frequency.
Some background in dielectric theory is required to understand the importance of this. FIG. 7 is the schematic representation for an ideal capacitor C, which does not actually exist. In this regard, all capacitors have varying degrees of undesirable resistance and inductance. This is explained in more detail in “A Capacitor's Inductance,” Capacitor and Resistor Technology Symposium (CARTS-Europe), Lisbon, Portugal, Oct. 19-22, 1999, the contents of which are incorporated herein.
FIG. 8 is a simplified equivalent circuit model of the capacitor. For the purposes of these discussions, the IR can be ignored as it is in the millions of ohms and does not significantly contribute to the capacitor equivalent series resistance (ESR). IR also has negligible effect on capacitor high frequency performance. The inductance (ESL) can also be ignored because inductive reactance for monolithic ceramic capacitors is very low at low frequencies. Inductance for a feedthrough capacitor is very low and can be thought of as negligible at high frequencies. Accordingly, the capacitor ESR is the sum of the dielectric loss, the ohmic losses and any losses due to skin effect. However, at low frequency, skin effect is negligible.
Therefore, a good low frequency model for capacitor ESR is as shown in FIG. 9. At low frequency, the capacitor ESR is simply the sum of the capacitor's ohmic and dielectric losses.
FIG. 10 illustrates a normalized curve which shows the capacitor equivalent series resistance (ESR) on the Y axis versus frequency on the X axis. This curve has been highly compressed into a U shape so that all of the important points can be illustrated on one graph. However, one should imagine FIG. 10 stretched out along its X axis by many times to get the true picture. The important point here is the dielectric loss is also known as the dielectric loss tangent. The dielectric material that is used to build the monolithic ceramic capacitor is in itself capable of producing real loss (resistance) which varies with frequency. The dielectric resistance is very high at low frequency and drops to zero at high frequency. This effect can be thought of as oscillations in the crystal structure that produce heat or changes in electronic or electron spin orbits that also produce heat. No matter which dielectric model one uses, this dielectric loss can be very significant at low frequency. In the EMI filter capacitor that's typically used in cardiac pacemakers and implantable defibrillators, a capacitance value of around 4000 picofarads is typical. Typical values of dielectric loss would be around 4000 ohms at 1 kHz, around 6 to 12 ohms at 1 MHz, and only a few milliohms at 10 MHz. This clearly indicates that as one goes up in frequency the dielectric loss tends to disappear.
Since the 1960s it has been a common practice in the capacitor industry to measure capacitance and dissipation factor at 1 kHz. The dissipation factor is usually defined as a percentage, for example, 2.5% maximum. What this means is that the dielectric loss resistance can be no more than 2.5% of the capacitive reactance at a certain frequency (usually 1 kHz). For example, if the capacitive reactance for a particular capacitor was 80,000 ohms at 1 kHz with a 2% dissipation factor this would equate to 1600 ohms of resistance at 1 kHz. FIG. 10 also illustrates that the dielectric loss essentially goes to zero at high frequency. For typical high dielectric constant monolithic ceramic capacitors, anything above 10-20 MHz will be sufficiently high in frequency so that the dielectric loss is no longer a factor in the capacitor ESR measurement. FIG. 10 also has superimposed on it another curve representing conductor ohmic loss which in a monolithic ceramic feedthrough capacitor is typically on the order of 0.25 ohms to 0.75 ohms. It should be pointed out that values of equivalent series resistance presented herein relate to only one illustrative example. In actual fact, the ESR of the capacitor varies with the capacitance value, the number of electrode plates, and the length and width of the electrode plates. Accordingly, a wide range of “normal” ESR readings can be obtained for many types of capacitors. For one particular capacitor a normal ESR reading might be 0.05 ohms and for another design as much as 10 ohms. The important thing is that the ESR reading and the lot population represent oxide free connections that are very homogenous and the readings are stable across the lot population.
It is also possible to detect those parts in a manufacturing lot population that for one reason or another have an abnormally high resistance reading. This can be done at 1 MHz by very tightly controlling the maximum allowable ESR. This is being done in the presence of relatively high dielectric loss. However, by holding a very tight screening limit it is still possible to detect such out of population part. This measurement is, of course, easier to do at 10 MHz, but also quite practical at 1 MHz.
The conductor ohmic losses come from all of the feedthrough capacitor conductor materials and connections. That would include the lead wire or circuit trace itself, the electrical connection between the lead wire and the capacitor metallization, which might be solder or a thermalsetting conductive adhesive, the interface between the capacitor metallization and the internal electrode plates, the connection from the capacitor ground metallization to a ferrule, and the bulk resistance of the electrode plates themselves. Conductor ohmic loss does not vary with frequency until skin effect comes into play. Skin effect is also shown on FIG. 10 and one can see that the resistance starts to climb at the higher frequencies. For physically small MLC chips and feedthrough capacitors, skin effect does not really play a role until one gets to very high frequencies, for example, above 200 MHz.
FIG. 11 is a more detailed illustration of the dielectric loss shown by itself. At very low frequency the dielectric loss in ohms is quite high and as frequency increases, one can see that dielectric loss tends to go to zero. On this scale, the conductor ohmic losses, which are shown as metal loss, can hardly be detected (these are only a few milliohms in this case).
As previously mentioned, titanium oxide (or niobium or tantalum oxides) can vary in resistance from a few milliohms all the way up to 10 or even 30 ohms. A recently discovered problem is that when one makes measurements at 1 kHz it is impossible to see the effects of these oxides because they are hidden by the dielectric loss tangent, which can be as high as 4000 ohms or more by itself. Trying to find a resistance that has increased from 0.25 ohms for a titanium surface that is free of oxide up to 2 ohms is literally impossible in the presence of 4000 ohms of dielectric loss. The reason for this is that the dielectric loss can vary slightly from part to part (typically plus or minus 20 percent). Therefore, when one is making measurements on a manufacturing lot of ceramic EMI feedthrough capacitors for medical implant applications, the part to part variation at 1 kHz can be as much as 100 ohms due to dielectric loss tangent variation alone. Therefore, it becomes quite impossible to detect the presence of this undesirable oxide layer on the titanium surface. However, the recently introduced Agilent equipment is capable of making dielectric equivalent series resistance measurements at 10 MHz and above. This is a high enough frequency to get rid of the dielectric loss so that one can see the ohmic loss by itself (without being hidden under the dielectric loss).
FIG. 12 is a sweep from the Agilent E4991A RF Impedance-Materials Analyzer. Curve 136 illustrates the capacitor equivalent series resistance vs. frequency. The presence of these oxides can reduce EMI filter performance by as much as 20 dB. Stated another way, this could reduce EMI filtering effectiveness by a ratio of 10 to 1 or more. This is highly undesirable in an implantable medical device given the previous documented clinical interactions between cellular telephones and pacemakers. For example, it has been shown that cellular telephone interference can completely inhibit a pacemaker or cause it to go into asynchronous tracking or other undesirable behavior. This can be very dangerous even life threatening for a pacemaker-dependent patient. Further compounding this concern is the recent introduction throughout the marketplace of cellular telephone amplifiers.
One example of this is in the off shore marine boating environment. Until recently maritime communications were primarily limited to the VHF radio. However, many boaters are now relying on cellular telephones for their communication. Accordingly, a number of companies have introduced cellular telephone amplifiers which boost cellular telephone output from 0.6 watts maximum to 3 watts. In addition, high gain marine antennas are being manufactured which can be anywhere from 4 to 8 feet long. These provide an additional 9 dB of gain in the extreme case. Passengers on these boats are being subjected to much higher field intensities than were previously contemplated by the FDA.
Another area where cellular telephone amplifiers are becoming increasingly popular is for wireless Internet connections for lap top computers. It is now possible to buy small black box devices that plug into the wall and also plug into the cellular telephone. These devices then plug into the lap top computer. This boosts the cellular telephone output to 3 watts and also provides a high gain antenna all of which sit on a desk top right in front of the operator. There are also remote credit card scanning devices that operate under similar principles. In short, the public is increasingly being exposed to higher levels of electromagnetic fields.
Accordingly, there is an urgent and present need for EMI filtered terminals for implantable medical devices that will not only maintain their present performance (by not degrading in the presence of oxides) but also increase in their performance. Co-bonded ferrite slabs are being contemplated in order to further increase filter performance in conjunction with the principles outlined here. This will allow future capacitor connections with very low ESR and very low potential for oxidation at attachment points. In addition, the additional ferrite slab will change it from a single element EMI filter to a double EMI filter (L filter). Accordingly, increased performance at cellular phone frequencies offered thereby providing complete immunity to the aforementioned new signal amplifiers. Returning to FIG. 12 one can see from the resistance curve 136 that at the far left hand side of the sweep (1) at 1 MHz, the resistance is approximately 6 ohms. This means that there is a significant, but small amount of dielectric loss tangents still present at 1 MHz (the dielectric loss tangent at 1 kHz is 1800 ohms). However, when one goes up to marker (2), which is at 10 MHz, we're at a point where the dielectric loss tangent has all but disappeared. At this point, we are primarily seeing the true ohmic losses of the device. The device measured in FIG. 12 has no titanium oxide build-up. Accordingly, at marker (2) we have a very low resistance measurement of 234.795 milliohms (0.234 ohms).
FIG. 13 is the same as the sweep in FIG. 12 except this is taken from a part that has a substantial amount of undesirable titanium oxide build-up. Curve 136 illustrates that at marker (2) there is 23.2529 ohms of resistance present. FIG. 13 clearly illustrates that there is enough titanium oxide build-up to create 23.2529 ohms of series resistance at 10 MHz (a normal reading is 0.234 ohms for this particular capacitor). This is highly undesirable because it will preclude the proper operation of an EMI filter at this frequency and frequencies above.
FIGS. 14-19 illustrate a prior art rectangular bipolar feedthrough capacitor (planar array) 200 mounted to the hermetic terminal 202 of a cardiac pacemaker in accordance with U.S. Pat. No. 5,333,095. Functionally equivalent parts shown in this embodiment relative to the structure of FIGS. 1-6 will bear the same reference number, increased by 100.
As illustrated in FIGS. 14-19, in a typical broadband or low pass EMI filter construction, a ceramic feedthrough filter capacitor, 200 is used in a feedthrough assembly to suppress and decouple undesired interference or noise transmission along one or more terminal pins 216, and may comprise a capacitor having two sets of electrode plates 206 and 208 embedded in spaced relation within an insulative dielectric substrate or base 204, formed typically as a ceramic monolithic structure. One set of the electrode plates 206 is electrically connected at an inner diameter cylindrical surface of the capacitor structure 200 to the conductive terminal pins 216 utilized to pass the desired electrical signal or signals (see FIG. 16). The other or second set of electrode plates 208 is coupled at an outer edge surface of the capacitor 200 to a rectangular ferrule 218 of conductive material (see FIG. 18). The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the capacitor 200.
In operation, the coaxial capacitor 200 permits passage of relatively low frequency electrical signals along the terminal pins 216, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors 200 of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (6) and additional lead configurations. Feedthrough capacitors 200 (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal such as titanium alloy, which is electrically and mechanically coupled to the hermetic terminal pin assembly which is in turn electrically coupled to the coaxial feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
FIG. 15 illustrates an unfiltered hermetic terminal 202 typical of that used in medical implant applications. The ferrule 218 is typically made of titanium or other biocompatible material. An alumina insulator 224 or other insulative material such as glass or the like, is used to electrically isolate the leads 216 from the conductive ferrule while at the same time providing a hermetic seal against body fluids. In the case of an alumina insulator, the lead wires or leads 216 are installed into the insulating material 224 typically by gold brazing. A gold braze is also formed between the alumina 224 and the ferrule 218. In some applications, this can also be done with sealing glass so that the gold brazes are not required. The reference numbers 228 and 230, on the one hand, and 228′ and 230′, on the other (FIG. 19), show gold brazes in two alternate locations that are used to form the hermetic seal between the titanium ferrule 218 and the alumina insulator 224.
FIG. 18 illustrates the capacitor 200 mounted to the hermetic terminal 202 of FIG. 15. The attachment 232 between the capacitor ground metallization 214 and the titanium ferrule 218 is typically done with a conductive thermalsetting polymer, such as conductive polyimide or the like. It is also required that an electrical/mechanical connection be made between the capacitor inside diameter holes 212 and the four lead wires 216. This is shown at 244 and can be accomplished with a thermalsetting conductive adhesive, solder, welding, brazing or the like.
FIG. 19 is a cross-sectional view of the capacitor assembly of FIG. 18, which is typical of prior art capacitors shown in U.S. Pat. No. 5,333,095 and related patents. In FIG. 19, one can see the undesirable oxide layer 234. This oxide layer can actually coat all surfaces of the titanium ferrule (for simplicity, it is only shown on FIG. 19 in the area where the conductive polyimide attachment 232 is made to the capacitor ground termination 214). The thermalsetting conductive material 232 connects between the capacitor ground metallization 214 and the ferrule 218. However, if there is an insulative titanium oxide layer 234 as shown, this can preclude the proper operation of the feedthrough capacitor 200 as previously mentioned.
From the foregoing it is seen that titanium housings, casings and ferrules for hermetic seals are commonly used in the medical implant industry. Pacemakers, implantable defibrillators, cochlear implants and the like, all have ferrules or housings made of titanium. All of the aforementioned devices are also subject to electromagnetic interference (EMI) from emitters that are commonly found in the patient environment. These include cell phones, microwave ovens and the like. There are a number of prior art patents which describe EMI feedthrough filters which make the implantable devices immune to the effects of EMI.
The presence of oxides of titanium can preclude the proper performance of monolithic ceramic EMI feedthrough filters. The titanium oxides that form during manufacturing processes or handling form a resistive layer, which shows up at high frequency. High frequency impedance analyzer plots of resistance vs frequency illustrate that this effect is particularly prominent above 10 MHz. There is a significant need, therefore, for a novel method of providing a conductive coating on the ferrules of human implantable hermetic seals for reliable EMI filter attachment. Further, there is a need for a novel method of electrical connection of feedthrough capacitor lead wire inside diameter termination directly to the gold termination or other similarly capable material of hermetic seals and corresponding lead wire(s). The present invention fulfills these needs and provides other related advantages.